The invention relates to coating agents for electrophotography carriers, particularly coating agents for electrophotography carriers which enable preparation of carrier particles with a stable charge amount, without being susceptible to the spent phenomenon, over a long duration as well as superior endurance.
As is well known, developers for the two component type dry copying machine contain two main particulate components: fine particulate toners and larger sized carrier particles. Mixing and stirring of these two components together causes friction and each toner particle and each carrier particle becomes electrostatically charged with different signs from each other. The visible image is formed by electrostatically depositing the charged toner particles on an electrostatic latent image formed on a photosensitive substance. Copying is completed by transferring this image to a transfer sheet and fixing it.
In this case, oxidized or unoxidized iron powders are usually used as carrier particles. However, when these carriers are utilized, as they are, by mixing with the toner, their triboelectric properties towards the toner are insufficient. In addition, during use, the toner particles tend to stick to the surface of the carrier particles forming a toner film thereon (which is called "spent phenomenon"). As a result, the charge characteristics of the carrier disadvantageously vary with time, and the lifetime of the developer becomes shortened. Further, another disadvantage is a large difference in the carrier charge characteristics under dry and humid conditions.
In order to prevent such demerits, carrier surfaces have been covered with resins such as acrylic resins, styrene-acryl copolymers, silicone resins, polyester resins and fluorine resins. However, in the case of using polyester resins having hydroxyl groups and acrylic resins as the coating, crosslinking is performed through the use of, for example, isocyanates and melamines. Incomplete crosslinking results in residual hydroxyl groups and isocyanate groups being present. This reaction progresses with time, which worsens the charge characteristics (Japanese Patent applications Kokai (laid open) 59-53875, and 60-59369).
Further, in the case of using acrylic resins and styrene-acryl copolymers as the coating, the initial charge characteristics are superior. However, the coating film tends to peel off from the particle nucleus due to poor adhesivity. On the other hand, the fluorine resins and silicone resins possess a low surface energy. Therefore, these coating agents are collecting attention from the viewpoint of reduced spent phenomenon (Japanese Patent applications Kokai 54-21730, 58-40557 and 59-131944, and Japanese Patent application Kokoku (examined) 59-26945; see also U.S. Pat. Nos. 4,514,485, 4,927,728, 5,068,301 and 5,368,969, and U.S. patent application Ser. No. 08/185,354 now U.S. Pat. No. 5,468,825).
Particularly, in the case of silicone resins, the selection of monomers leads to variety in the molecular structures. Therefore, their selection enables one to prepare carriers with various levels in the amount of charge. In addition, the silicone resins are soluble in many solvents, which leads to many working advantages such as a uniform coating on the carrier surface and curing at relatively low temperature.
On the other hand, a method has been proposed to control the charge amount by the silicone resins having tin compounds (Japanese Patent applications Kokoku 2-3181). This method, however, increases the charge amount disadvantageously as the number of copies made is increased.